Physics 101: Lecture 11 Momentum and ImpulsePhysics 101: Lecture 9, Pg 12 Momentum ACT A car w/ mass...
Transcript of Physics 101: Lecture 11 Momentum and ImpulsePhysics 101: Lecture 9, Pg 12 Momentum ACT A car w/ mass...
Physics 101: Lecture 9, Pg 1
Physics 101: Lecture 11 Momentum and Impulse
Today’s lecture will cover Textbook Sections 7.1-7.5
Exam II
Physics 101: Lecture 9, Pg 2
Comments:
Exam tonight – 7 PM (5:15 PM conflict)
Free points for lecture and checkpoint!
Physics 101: Lecture 9, Pg 3
Momentum and Impulse
Momentum p = mv
» Momentum is a VECTOR
Impulse I = FDt
» Impulse is just the change in momentum: I = Dp
Physics 101: Lecture 9, Pg 4
Key Ideas
Earlier: Work-Energy
FNet = m a multiply both sides by d
FNet d= m a d (note a d = ½ Dv2)
FNet d= ½ m Dv2
WNet = DKE Define Work and Kinetic Energy
This Time: Impulse-Momentum
FNet = m a
FNet = m Dv/Dt (note a = Dv/Dt )
FNet = D (mv)/Dt = Dp/Dt
INet = Dp Define Impulse and Momentum demo
Physics 101: Lecture 9, Pg 5
Checkpoint 1 A magician yanks a tablecloth out from under some dishes. Miraculously, the dishes remain on the table where they were. Briefly explain why the dishes were not given much impulse by the tablecloth.
Impulse is force multiplied by time, so the dishes were not given much impulse since the time was so small.
Uh...magic. DUH. … Good thing we know that physics is all gibberish and the real answer to these questions is magic.
The cloth must be silky smooth.
the magician is not as 'magical' as he seems-- like the members of his audience, he is a slave to the laws of physics…
Physics 101: Lecture 9, Pg 6
ACT (w/ demo)
Two identical balls are dropped from the same height onto the
floor. In each case they have velocity v downward just before
hitting the floor. In case 1 the ball bounces back up, and in
case 2 the ball sticks to the floor without bouncing. In which
case is the magnitude of the impulse given to the ball by the
floor the biggest?
A. Case 1
B. Case 2
C. The same
correct
Bouncing Ball
|I| = |Dp|
= |mvfinal – m vinitial|
= |m( vfinal-vinitial)|
= 2 m v
Sticky Ball
|I| = |Dp|
= |mvfinal – m vinitial|
= |m( 0-vinitial)|
= m v
Vi = -5 m/s Vf = +5 m/s
Physics 101: Lecture 9, Pg 7
ACT
In both cases of the above question, the
direction of the impulse given to the ball by the
floor is the same. What is this direction?
A. Upward
B. Downward
time
correct
Physics 101: Lecture 9, Pg 8
You drop an egg onto 1) the floor 2) a thick
piece of foam rubber. In both cases, the egg
does not bounce (demo).
In which case is the impulse greater?
A) Floor
B) Foam
C) the same
In which case is the average force greater
A) Floor
B) Foam
C) the same
ACTs
I = DP
Same change in momentum
Dp = F Dt
F = Dp/Dt
Smaller Dt = large F
Physics 101: Lecture 9, Pg 9
Pushing Off… Fred (75 kg) and Jane (50 kg) are at rest on skates facing each other. Jane then pushes Fred w/ a constant force F = 45 N for a time Dt=3 seconds. Who will be moving fastest at the end of the push?
A) Fred B) Same C) Jane
Fred
F = +45 N (positive direct.)
I = +45 (3) N-s = 135 N-s
I = Dp
= mvf – mvi
I/m = vf – vi = vf
vf = 135 N-s / 75 kg
= 1.8 m/s
Jane
F = -45 N Newton’s 3rd law
I = -45 (3) N-s = -135 N-s
I = Dp
= mvf – mvi
I/m = vf - vi = vf
vf = -135 N-s / 50 kg
= -2.7 m/s
Note: Pfred + Pjane = (1.8) 75 + (-2.7) 50 = 0!
Physics 101: Lecture 9, Pg 10
Momentum is Conserved
Like Energy, Momentum is “Conserved”, meaning it can not be created nor destroyed
Can be transferred
Total Momentum does not change with time.
This is a BIG deal!
Physics 101: Lecture 9, Pg 11
Impulse and Momentum Summary
FNetDt = Dp
For single object….
FNet = 0 momentum conserved (Dp = 0)
For collection of objects …
FNet = 0 total momentum conserved (DPtot = 0)
FNet = mtotal a
Physics 101: Lecture 9, Pg 12
Momentum ACT A car w/ mass 1200 kg is driving north at 40 m/s, and turns
east driving 30 m/s. What is the magnitude of the car’s change in momentum?
A) 0 B) 12,000 C) 36,000 D) 48,000 E) 60,000
Pinitial = m vinitial = (1200 Kg) * 40 m/s = 48,000 kg m/s North
Pfinal = m vfinal = (1200 Kg) * 30 m/s = 36,000 kg m/s East
North-South:
Pfinal – Pinitial = (0 – 48000) = -48,000 kg m/s
East-West:
Pfinal – Pinitial = (36000 - 0) = +36,000 kg m/s
Magnitude :
Sqrt(P2North + P2
East ) = 60,000 kg m/s
40 m/s
30 m/s
Physics 101: Lecture 9, Pg 13
Checkpoint 2 & 3
Is it possible for a system of two objects to have zero total
momentum while having a non-zero total kinetic energy?
1. YES
2. NO “two identical cars, say Porsche's, running towards each other at identical speeds … … … … and if you are Bruce Willis, it may take out a helicopter..”
correct
“an explosion! BOOM.”
“If Superman and Bizarro were flying dirrectly at each other in the vacuum of space, they would both have scalar kinectic energy, and equal and opposite momentum.” demo
Physics 101: Lecture 9, Pg 14
ACT
Movies often show someone firing a gun loaded with
blanks. In a blank cartridge the lead bullet is removed
and the end of the shell casing is crimped shut to
prevent the gunpowder from spilling out. When a gun
fires a blank, is the recoil greater than, the same as, or
less than when the gun fires a standard bullet?
A. greater than
B. same as
C. less than
If there is no bullet then pbullet = 0 so pgun = 0
correct pgun = -pbullet
As if ice skater had no one to push…
Physics 101: Lecture 9, Pg 15
Summary
Impulse I = FDt
» Gives change in momentum I = Dp
Momentum p = mv
» Momentum is VECTOR
» Momentum is conserved (when FNet = 0)
S mvinitial = S mvfinal